Piezoelectric devices for thin-film electronics have a broad range of frequency responses, from DC force sensors and scales to piezoacoustic or acousto-electric sensors (AES), ultrasonic transducers and PWAS (piezoelectric wafer-active sensor) devices, which are used for high-frequency defect scanning, materials testing, impedance measurements and structural monitoring. In the area of piezoelectric crystal oscillators, response frequencies extend even higher, to the megahertz (MHz) range and above.
Piezoelectric materials are electrically neutral but have an anisotropic charge distribution, which results in a net polarization when the material is deformed. The polarization field generates a piezoelectric voltage (or a current signal), which varies as a function of the applied mechanical stress or strain. Alternatively, an external field can be applied in order to deform the piezoelectric, producing mechanical effects such as flexing an armature, manipulating a microelectromechanical system (MEMS) or microactuator device, or producing an acoustic, ultrasonic, or radio-frequency (RF) vibration.
Piezoelectric effects were first identified in natural single-crystal structures such as quartz, gallium phosphate and tourmaline. Modern industrial applications, however, typically utilize synthetic polycrystalline or sintered piezoceramic materials such as aluminum nitride (AlN), barium nitride (BN) and barium titanate (BaTiO3). These materials can be manufactured in almost any shape and size, and the composition and manufacturing techniques can be varied in order to scale the piezoelectric effect to meet particular engineering requirements.
In synthetic polycrystalline materials, the piezoelectric effect depends upon the orientation of individual dipole regions within the material, which are referred to as Weiss domains. In general the Weiss domains are randomly oriented when the piezoelectric is formed, but they can be aligned by poling the material in an electric field, typically at elevated temperature. Poling encourages the growth of domains oriented along the poling field direction, and tends to reverse the orientation of anti-parallel domains.
Poling also reorients space charges and aligns the remnant polarization in ferroelectric materials such as Pb[ZrxTi1-x]O3 (or PZT) based materials, which are inherently piezoelectric due to symmetry considerations. Essentially, poling reduces randomization in the domain orientations, generating a bulk domain asymmetry to yield a net piezoelectric effect. The piezoelectric/ferroelectric film thickness can also be varied, in order to enhance response for particular electronics applications.
Poled piezoelectric materials can actually have greater piezoelectric response than single-crystal (naturally-occurring) materials, and offer much more flexibility in manufacturing. Unfortunately, poling can also be expensive and time consuming, particularly when large numbers of individual devices are involved. As a result, there is a constant need for cost-effective and efficient poling techniques, as applicable to volume manufacturing for a wide range of different piezoelectric devices, including piezoacoustic sensors, ultrasonic transducers, MEMS devices, and other piezoelectric-based thin-film electronics and semiconductor components.